There has been known a technique in which the degradation of an oxidation catalyst arranged upstream of a particulate filter for trapping particulate matter (PM) in an exhaust gas is determined based on a difference between the temperature of the exhaust gas flowing into the oxidation catalyst and the temperature of the exhaust gas flowing out of the oxidation catalyst after a predetermined period of time has elapsed from the start of supply of unburnt fuel for regenerating the filter (see, for example, a first patent document).
However, the deterioration of a catalyst tends to progress earlier at a more upstream side thereof. That is, the deterioration of the catalyst progresses from an upstream side portion of the catalyst toward a downstream side portion thereof. Here, when the extent of the deterioration of the catalyst is still low, the temperature of the exhaust gas at the downstream side from the catalyst may become higher, as compared with the catalyst which has not deteriorated.
Here, with the catalyst which has not deteriorated, unburnt fuel reacts and generates heat at the upstream side portion of the catalyst. Then, this heat flows toward the downstream side together with the exhaust gas, so that the heat is provided to the downstream side portion of the catalyst. That is, due to the reaction of the unburnt fuel in the upstream side portion of the catalyst, heat is provided to a wide range of the catalyst. For this reason, a lot of heat is given to the catalyst. As a result of this, the temperature of the catalyst as a whole goes up, and at the same time, the temperature of the exhaust gas flowing out of the catalyst becomes low.
On the other hand, in cases where the extent of the deterioration of the catalyst is low, unburnt fuel mainly reacts in the downstream side portion thereof. This reaction heat provides heat to the downstream side portion of the catalyst. However, the heat is generated in the downstream side from the catalyst, so there is a small amount of heat given to the upstream side portion of the catalyst. As a result, the heat carried by the exhaust gas flowing out of the catalyst increases more, as compared with the catalyst which has not deteriorated, and hence, the temperature of the exhaust gas at the downstream side form the catalyst becomes temporarily high. That is, in cases where the catalyst has deteriorated to some extent, the temperature of the exhaust gas at the downstream side from the catalyst may become high. Then, as the deterioration of the catalyst progresses further, unburnt fuel becomes difficult to react in the downstream side portion of the catalyst, so the temperature of the exhaust gas at the downstream side from the catalyst becomes lower, as compared with the catalyst which has not deteriorated.
Thus, in cases where the catalyst has deteriorated, the temperature of the exhaust gas at the downstream side from the catalyst may become high or low. As a result, if the deterioration of the catalyst is determined based on a difference between the temperature of the exhaust gas at the upstream side from the catalyst and the temperature thereof at the downstream side from the catalyst, as in the past, there will be a fear that the accuracy of the determination may become low.
In addition, in the past, the determination of deterioration of the catalyst has been carried out during the steady state operation of an internal combustion engine, but at the time of the steady state operation, there is not so much difference in the amount of reaction of the unburnt fuel between a catalyst of which the extent of the deterioration is low and a catalyst which has not deteriorated. For this reason, if the deterioration determination of the catalyst is going to be carried out at the time of a steady state operation, it has to be carried out at the time of a limited operating state in which an appreciable or obvious difference appears in the amount of reaction of unburnt fuel. As a result, the opportunity for the deterioration determination of the catalyst decreases, so the deterioration determination of the catalyst takes much time.